Domestic appliance



Nov. 22, 1960 G. a. LONG DOMESTIC APPLIANCE Filed April 2, 1957 llWf/VTOR GfOfi GE B. LONG 5 y W Hi5 ATTOBNE United States Patent DOMESTIC APPLIANCE George B. Long, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Apr. 2,1957, Ser. No. 650,259 Claims. (Cl. 219-1055) This invention relates to a domestic appliance and more particularly to domestic electric ovens in which the electromagnetic high frequency waves of the order of 915 or 2500 megacycles are used to rapidly cook foods.

Such ovens have the capability of cooking foods in a minute or two, but a number of difficulties have been encountered in addition to teaching this new method of cooking. One of the ditficulties has been that parts of a food load such as a roast or a cake are well-done or overdone, while other parts are underdone. Allegedly, this is due to the presence of hot spots and cold spots within the oven. Attempts have been made to combat this difiiculty with stirrers, rotating antennas and various forms of wave guides. While these procedures provide means for reducing the difficulties, they have not eliminated them.

It is an object of this invention to obtain even and uniform cooking and baking in an electronic oven, particularly in the region wherein roasts and cakes are normally baked. i

It is another object of this invention to provide a more even voltage. gradient throughout the region of the food to be cooked or baked. i i

It is another object of this invention to provide a simple means within an electronic oven which will shift and alter the wave pattern and eliminate hot and cold spots.

It is another object of this invention to provide a simple, inexpensive improved metal spring gasket readily applied to and retained upon the door jamb for grounding the door to the door jamb for preventing arcing and also for preventing the escape of radio frequency Waves.

These and other objects are attained in the form of the invention shown in the drawing in which the microwave cooking device is provided with an electron discharge device of the magnetron type located beneath the oven provided with a rotating antenna driven by the blower motor. Above the rotating antenna is a horizontal bar-type metal shelf provided with relatively widely spaced bars, which is supported upon guideways of electrical insulating material mounted upon the sidewalls of the oven. This shelf may support materials to be heated, cooked, baked or roasted. Spaced above the bar shelf is a second horizontal shelf of regular plate glass which may be supported on similar guideways of electrical insulating material fastened to the side walls of the oven. This sheet of plate glass may also be used to support material to be heated, cooked, baked or roasted. The plate glass is not transparent to the radio frequency energy but has an appreciable loss and a dielectric constant above 7. It acts to diffuse and change the wave pattern within the oven to reduce the hot spots and cold spots.

Further objects and advantages of the present inven- 2,961,520 Patented Nov. 22, 1960 tion will be apparent from the following description, reference being had to the accompanying drawings, wherein a preferred form of the present invention is clearly shown.

In the drawings:

Figure 1 is a front view, partly in section and partly diagrammatic, showing a microwave oven embodying one form of my invention; and

Figure 2 is a horizontal fragmentary sectional view taken along the line 2-2 of Figure 1.

Referring now to the drawing, there is shown a micro wave oven 20 provided with a metal enclosure 22 enclosing the oven compartment 23. The metal enclosure 22 has an outwardly curled flange 24 which holds the curled inner edge 26 of a thin spring metal gasket 28. The curled inner edge of the gasket is held firmly beneath the outwardly curled flange 24. The gasket 28 is provided with an outwardly bulged portion 30 connecting with the inner curled portion 26 and having rearwardly extending tongues 32 extending through slots 34 in the door-jamb 36. The door 38 closes against the outwardly bulged portion 30 which yields to the high spots so that a substantially tight contact is made throughout with the inner face of the door. On the front wall of the oven above the oven compartment 24 are the various control 40.

Beneath the oven compartment there is provided a compartment 42 for the electron discharge device 44 of 915 megs. This device is of the magnetron type and includes a coaxial transmission line 46 and a double outlet blower 48 having an outlet 50 discharging below the electron discharge device 44, and an upper discharge outlet 52 discharging into the transmission line 46 above the discharge device 44. The transmission line 46 includes an inner conductor 54 extending up into the hollow bottom of the rotating conductor 56. The rotating conductor 56 is rotatably mounted upon the lower and upper insulating bearings 58 and 60 provided within the transmission line 46. The blower 48 is provided with a small pulley 62 which, through the belt 64 and the selectively located pulley 66, rotates the conductor portion 56. The conductor portion 56 and the coaxial transmission line 46 extend through an aperture in the bottom wall of the metal enclosure 22. Within the oven compartment the conductor portion 56 is provided with a rotating antenna 68 having arms of a quarter and a half wavelength.

Spaced above the antenna 68 is a horizontal shelf 70 made of parallel steel bars fastened to a perimeter bar. This shelf 70 is slidably supported in the guides 72 fixed to the side walls of the metal enclosure 22. These guides 72 are of low loss electrical insulating material and of such dimension and location so as to prevent arcing between the metal shelf 70 and the metal enclosure 22. The side walls of the enclosure 22 are provided with similar guides 74, 76 and 78 spaced at different heights above the guides 72 within the oven. The bar shelf 70 is a matching device to electromagnetically match, radiate, distribute and make uniform the electromagnetic waves. However, I find that there are hot spots and cold spots within the oven. According to my invention, spaced above the bar-type shelf 70 is a shelf 80 of regular A plate soda lime glass which has a higher loss and different properties than quartz and high quartz glass which are practically transparent to radio frequency waves. It has a composition containing less than of SiO This has a dielectric constant between 7 and 7%. It has a relatively high loss factor (which varies with the frequency in megacycles) as high as .059. The average composition may be as follows:

I find that this plate glass shelf gets only slightly warm from the wave energy it absorbs. It, however, does appear, if properly located, to minimize the hot and cold spots and apparently provides a more even voltage gradient and more uniform wave distribution. It also seems to prevent cancellation in same areas. For example, a hot spot pattern in a food load can be shifted by the introduction of this material. Perhaps it performs as an impedance matching medium to limit intense microwave field concentration. The glass shelf 80 may be used to support a reinforced polyester pan 82 containing the material 84 for baking a cake. The shelf 70 may also support a receptacle 86 containing material to be heated, baked or roasted. The plate glass shelf 80 appears to have an effect upon the wave pattern both above and below. The plate glass 80 may be placed also vertically or at various angles and positions to obtain the uniformity and distribution desired.

While the form of embodiment of the invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted, as may come within the scope of the claims which follow.

What is claimed is as follows:

1. A microwave cooking device including a metal enclosure, means for radiating radio frequency energy into the interior of said enclosure, a metal shelf having openings therein within said enclosure, electrical insulating means between the adjacent portions of said shelf and said enclosure, and a sheet of glass having an Si content of less than A and a relatively high loss factor extending between the walls of said enclosure for changing the Wave pattern of the radio frequency energy within said enclosure.

2. A microwave cooking device including a metal enclosure, means for radiating radio frequency energy into the interior of said enclosure, and a sheet of soda lime glass having an SiO content of less than A and a relatively high loss factor extending between the walls of said enclosure with the central portions of the sheet spaced away from the walls for changing the wave pattern of the radio frequency energy within said enclosure.

3. A microwave cooking device including a metal enclosure, means for radiating radio frequency energy into the interior of said enclosure, and a sheet of soda lime glass having an SiO content of less than and a relatively high loss factor extending between the walls of said enclosure for changing the wave pattern of the radio frequency energy within said enclosure, said sheet of glass extending horizontally for supporting material to be cooked.

4. A microwave cooking device including a metal enclosure, means for radiating radio frequency energy into the interior of said enclosure, a metal shelf having openings therein within said enclosure, electrical insulating means between the adjacent portions of said shelf and said enclosure, and a sheet of glass having an Si0 content of less than and a relatively high' loss factor extending between the walls of said enclosure for changing the wave pattern of the radio frequency energy within said enclosure, said radiating means being located at the bottom of said enclosure, said shelf and glass being located in spaced horizontal planes located one above the other.

5. A microwave cooking device including a metal enclosure, means for radiating radio frequency energy into the interior of said enclosure, a metal shelf having openings therein within said enclosure, and a sheet of soda lime glass having Si0 content of less than and a relatively high loss factor extending between the walls of said enclosure for changing the wave pattern of the radio frequency energy within said enclosure.

References Cited in the file of this patent UNITED STATES PATENTS 2,714,070 Welch July 26, 1955 2,762,893 Long et a1. Sept. 11, 1956 2,778,911 Valentine Jan. 22, 1957 2,782,292 Long Feb. 19, 1957 2,827,537 Haagensen Mar. 18, 1958 2,831,952 Warner Apr. 22, 1958 FOREIGN PATENTS 752,206 Great Britain July 4, 1956 

